Circuit for detecting low-power optical data signal

ABSTRACT

A circuit for detecting an optical data signal includes a photonics substrate and first and second photodiodes formed in the photonics substrate. The first photodiode is configured to receive, via an input port formed in the photonics substrate, a first portion of the optical data signal and convert light power of the first portion of the optical data signal to generate a first current based on the optical data signal. The second photodiode is configured to output a second current without receiving any portion of the optical data signal. The second current corresponds to a dark current induced in the second photodiode. The circuit is configured to subtract the second current from the first current to generate an output signal corresponding to a power of the optical data signal without dark current induced in the first photodiode.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 17/012,629, filed on Sep. 4, 2020. The entire disclosure of theapplication referenced above is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to optical communication device. Moreparticularly, the present invention provides a method for makinghigh-sensitivity photodiode pair for detecting low-power optical signalin silicon photonics platform.

Over the last few decades, the use of broadband communication networksexploded. In the early days Internet, popular applications were limitedto emails, bulletin board, and mostly informational and text-based webpage surfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. Additionally, Internet of Things certainly will create evenhigher demand on data communication. With such high demands on data anddata transfer, existing data communication systems need to be improvedto address these needs.

As science and technology are updated rapidly, processing speed andcapacity of the computer increase correspondingly. With the advances ofoptical communication technology and applications driven by the marketdemand on increasing bandwidth and decreasing package footprint, moreintensive effort and progress have been seen in the development ofsilicon photonics. With its low-cost and CMOS compatible fabricationprocess, integration of electro-photonic circuits insilicon-on-insulator (SOI) substrate for forming high-speedhigh-data-rate silicon photonics devices has continuously gaining itsmarket in broadband optic-electric communication system.

With the increasing demand of high bandwidth and high integrability inoptical communication system, the optical components are more and moreintegrated in silicon photonics substrate with reducing devicedimensions, resulting a work environment with low-power optical signalsand quickly increasing temperature. For example, photodiodes made insilicon photonics platform need to be optimized for detecting low-poweroptical signal with enhanced sensitivity. However, existing siliconphotonics based photodiodes on SOI substrate can get their dark currentincrease as much as 8 times when temperature increases from roomtemperature to 95° C. at a bias voltage of merely −2V and as much as 4times over their device lifetime. Therefore, improvement on detectinglow-power optical signal with high sensitivity in silicon photonicssystem is needed.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to optical communication device. Moreparticularly, the present invention provides a method for detectinglow-power optical signal in silicon photonics platform. Merely byexample, the present invention discloses a method for making aphotodiode pair in a close-neighborhood of a silicon photonics substrateto detect one or more branches of input optical signals in low-powerwith high-sensitivity, though other applications are possible.

In a specific embodiment, the present invention provides a method formaking a pair of photodiodes to detect low-power optical signal in asilicon photonics integrated circuit. The method includes providing awaveguide in a silicon photonics substrate to deliver an input opticalsignal to the silicon photonics integrated circuit. The waveguideincludes one or more branches in the silicon photonics substrate.Additionally, the method includes forming a pair of nearly redundantphotodiodes in silicon photonics platform in the silicon photonicssubstrate. The method further includes coupling a first one of the pairof nearly redundant photodiodes optically to each of the one or morebranches for receiving the input optical signal combined from all of theone or more branches. Furthermore, the method includes coupling a secondone of the pair of nearly redundant photodiodes electrically in seriesto the first one of the pair of nearly redundant photodiodes. Moreover,the method includes drawing a current from the first one of the pair ofnearly redundant photodiodes under a reversed bias voltage applied tothe pair of nearly redundant photodiodes.

In another specific embodiment, the present invention provides a methodfor detecting a low-power optical signal in a waveguide of a siliconphotonics system. The method includes coupling a first photodiode to ataping output port of the waveguide to absorb a certain portion of powerof the low-power optical signal. Additionally, the method includesproviding a second photodiode that is nearly redundant to the firstphotodiode excluded from any power of the low-power optical signal. Themethod further includes applying a reversed bias voltage commonly to thefirst photodiode and the second photodiode. Furthermore, the methodincludes drawing a first current from the first photodiode under thereversed bias voltage including a photocurrent converted from thecertain portion of power of the low-power optical signal and a firstdark current. The method further includes drawing a second current fromthe second photodiode under the reversed bias voltage including a seconddark current. Moreover, the method includes performing a firmwareprocess to subtract the second current from the first current.

Many benefits can be achieved with the present invention on a method fordetecting low-power optical signal received in silicon photonicscircuit. The method takes advantage of existing silicon-photonics-basedphotodiode (PD) pair formed in a close-neighborhood in a SOI substrateto yield nearly redundant physical properties. The photodiode pairincludes a main PD coupled in series, cathode to anode, with a dummy PD.The main PD is configured to couple one or more branches of an inputwaveguide in the SOI substrate for detecting low-power optical signalswith at least doubled sensitivity. The dummy PD is used to cancel darkcurrent in the main PD to substantially reduce noise of photocurrentdrawn from the main PD. The invention effectively increases powermonitoring accuracy with minimum alternation in existing deviceformation process by at least doubling the sensitivity and minimizingdark current variation. The invention provides a simple way fordetecting low-power optical signal with high sensitivity especially forthe photodiodes in silicon photonics modules working at environmenttemperature as high as 95° C. for lifetime over 8000 hrs.

The present invention achieves these benefits and others in the contextof CMOS-compatible process for fabricating silicon photonics devices.However, a further understanding of the nature and advantages of thepresent invention may be realized by reference to the latter portions ofthe specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a schematic diagram of a photodiode being used conventionallyin a silicon photonics circuit for detecting an optical signal deliveredvia a waveguide in a silicon photonics substrate setting.

FIG. 2 is a simplified diagram of two photodiodes being usedconventionally in a silicon photonics circuit for respectively detectingtwo split optical signals delivered via a waveguide in a siliconphotonics substrate setting.

FIG. 3 is a schematic diagram of a close-neighbor photodiode pair in asilicon photonics circuit being configured to detect a low-power opticalsignal delivered via a waveguide in a silicon photonics substratesetting according to an embodiment of the present disclosure.

FIG. 4 is a schematic diagram of a close-neighbor photodiode pair in asilicon photonics circuit being configured to detect a low-power opticalsignal delivered via a waveguide with two or more split branches in asilicon photonics substrate setting according to an embodiment of thepresent disclosure.

FIG. 5 is a schematic diagram of a close-neighbor photodiode pair in asilicon photonics circuit being configured to detect a low-power opticalsignal delivered from two or more split branches of waveguide accordingto another embodiment of the present disclosure.

FIG. 6 is a flow chart showing a method of configuring a nearlyredundant photodiode pair for detecting a low-power optical signal withhigh sensitivity according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to optical communication device. Moreparticularly, the present invention provides a method for detectinglow-power optical signal in silicon photonics platform. Merely byexample, the present invention discloses a method for making aphotodiode pair in a close-neighborhood of a silicon photonics substrateto detect one or more branches of input optical signals in low-powerwith high-sensitivity, though other applications are possible.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,main, dummy, close-neighbor, reverse, forward, series, parallel, havebeen used for convenience purposes only and are not intended to implyany particular fixed direction. Instead, they are used to reflectrelative locations and/or directions between various portions of anobject.

FIG. 1 is a schematic diagram of a photodiode being used conventionallyin a silicon photonics circuit for detecting an optical signal deliveredvia a waveguide in a silicon photonics substrate setting. As shown, in asilicon photonics substrate 10, a portion of a silicon photonicsintegrated circuit includes an input port 100 for receiving an incomingoptical signal. Optionally, the optical signal is delivered via awaveguide 101 formed in the silicon photonics substrate 10. In aconventional setting for detecting or monitoring the optical signal, atap coupler 300 is coupled to the waveguide 101 to split a small portion(e.g., 5%) of the optical signal to a taping output port 103 whilekeeping major portion (95%) of the optical signal in the waveguide 102.The conventional setting for detecting or monitoring the optical signalfurther includes a silicon-photonics-based photodiode (PD) 400 coupledto the taping output port 103. Optionally, the PD 400 is configured toconvert (absorb) light of the optical signal to a photocurrent 410 asthe photodiode reversely biased. Measuring the photocurrent 410 leads todetection of the optical signal. Optionally, the photodiode PD 400 isformed in silicon photonics platform directly built in the siliconphotonics substrate 10. Optionally, the photodiode PD 400 is a Ge-on-Siphotodiode having a germanium absorption layer on top of a silicondevice layer of a silicon-on-insulator (SOI) substrate. Optionally, theGe-on-Si photodiode is configured to detect optical signal transmittedwith high data rate. However, the photodiode 400 may have relativelyhigh or increasing dark current on top of the photocurrent convertedfrom the optical signal, introducing higher noise and lowersignal-detection sensitivity under this conventional setting especiallywhen the optical signal has a low optical power while the PD 400 worksin a high-temperature (e.g., up to 95° C.) environment over itslifetime.

FIG. 2 is a simplified diagram of two photodiodes being usedconventionally in a silicon photonics circuit for respectively detectingtwo split optical signals delivered via a waveguide in a siliconphotonics substrate setting. As shown, a portion of silicon photonicsintegrated circuit receives an input optical signal coming from an inputport 200 led into a waveguide 201 formed in a silicon photonicssubstrate 10. The silicon photonics integrated circuit includes asplitter 250 coupled to the waveguide 201 to split the input opticalsignal to two branches of waveguide 251 and 252. Optionally, theincoming optical signal is a polarization-sensitive signal including amixture of Transverse electric (TE) mode and Transverse magnetic (TM)mode. Since the photodiode is generally adapted for detecting opticalsignal in TE mode. The partial signal in TM mode needs to be split fromthe input optical signal and be subjected to a polarization rotationoperation to convert to a partial signal in TE mode. Referring to FIG. 2, the splitter 250 optionally is a polarization-rotation splitter whichoutputs TE mode partial signal to each split branch while one of the TEpartial signal represents actual TM partial signal in original inputoptical signal.

In order to monitor the power level P of the input optical signal in thewaveguide 201 that is, for the reason mentioned above, split to twopartial signals respectively in two branches of waveguide, a firstbranch 251 and a second branch 252. An existing method is to couple onephotodiode (PD) to a respective one of the two branches via one tapcoupler. Referring to FIG. 2 , a first PD 401 is coupled to a tapingoutput port 255 of a first tap coupler 301 connected to the first branch251 to tap 5% power of a first partial signal thereof and a second PD402 is coupled to a taping output port 256 of a second tap coupler 302connected to the second branch 252 to tap 5% power of a second partialsignal thereof. The first PD 401 is configured to convert the firstpartial signal into a detectable photocurrent I_(ph1) drawn from anode411. By measuring the photocurrent I_(ph1) at the anode 411 of the firstPD 401 a power level P1 of the first partial signal is determined. Thesecond PD 402 is configured to convert the second partial signal intoanother detectable photocurrent I_(ph2) drawn from anode 412. Bymeasuring the photocurrent I_(ph2) at the anode 412 of the second PD 402a power level P2 of the second partial signal is determined. Inprinciple, the power of the input optical signal, P=P1+P2, can bedetected or monitored.

However, the current drawn from the anode 411 or 412 also includes adark current associated with the respective one PD 401 or 402.Optionally, the PD 401 or 402 is provided with Ge photodiode formed inSOI substrate. For a single Ge Photodiode on SOI substrate, its darkcurrent I_(dk) can increase as much as 8 times when temperatureincreases from room temperature to 95° C. with a bias voltage of −2V.Additionally, the dark current I_(dk) can increase as much as 4 times ata bias voltage of −2V over its 8000 hours lifetime. The significantchanges of the dark current of PD over temperature and over lifetimewill impact the sensitivity of detecting low-power optical signal.

FIG. 3 is a schematic diagram of a close-neighbor photodiode pair in asilicon photonics circuit being configured to detect a low-power opticalsignal delivered via a waveguide in a silicon photonics substratesetting according to an embodiment of the present disclosure. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in an embodiment,a pair of close-neighbor photodiodes, PD1 and PD2 formed under a sameCMOS process on the silicon photonics substrate 10, is configured to becoupled in a similar silicon photonics circuit setup like FIG. 1 fordetecting an input optical signal in low-power received from an inputport 100. Optionally, the silicon photonics substrate 10 is provided asa silicon-on-insulator (SOT) substrate having a silicon device layer ona buried oxide (BOX) layer.

In the embodiment, the same CMOS process for forming the pair of PDs inclose neighboring distance din the silicon photonics substrate 10, if dis limited to about 500 nm or less, would lead to two PDs with nearlyredundant physical properties. Among the pair of PDs, a first photodiodePD1 441 is a main PD that is optically coupled to a taping output port103 of a tap coupler 300 in the waveguide 101 connected to the inputport 100. A second photodiode PD2 442 is a dummy PD that is notoptically coupled to the waveguide 101 but electrically coupled inseries, cathode to anode, to the main PD 441 or PD1. Thus, the main PD441 serves a light-absorption/conversion device but the dummy PD 442does not. Under reversed bias condition and the serial connectionbetween the main PD and the dummy PD, the current drawn from the anodeport 451 of the main PD 441 will be I=I_(ph)+I_(dk1)−I_(dk2), whereI_(ph) is the photocurrent induced by the light conversion in the mainPD 441 and I_(dk1) and I_(dk2) are respective dark current in the mainPD and dummy PD 442. The physical properties of thesilicon-photonics-based PDs, e.g., Ge-on-Si photodiodes, are dependedupon the pattern layout of the absorption layer (Ge) and dimensions ofPIN junction structure at least partially in the silicon device layer ofthe SOT substrate. These physical properties, such as the dark currentin the Ge absorption layer, can be made substantially the same during asame CMOS fabrication process since the pair of photodiodes is locatedin close proximity with a distance d. For example, the distance d can beas smaller than 500 nm. Therefore, Icier I_(dk1)≈I_(dk2) and the currentdrawn from the anode port 451 of the main PD 441 will be nearly purephotocurrent I=I_(ph) with the dark current of the main PD 441 almostbeing canceled by the dark current of the dummy PD 442. Thissubstantially reduced noise in measurement of photocurrent I_(ph) andenhances photo-detection sensitivity for the silicon-photonics-basedphotodiode to detect low-power or even extremely-low-power opticalsignal delivered in the silicon photonics integrated circuit. In fact,the photo-detection sensitivity of the pair of photodiodes inclose-neighbor positions nearly is doubled compared to a singlephotodiode.

FIG. 4 is a schematic diagram of a close-neighbor photodiode pair in asilicon photonics circuit being configured to detect a low-power opticalsignal delivered via a waveguide with two or more split branches in asilicon photonics substrate setting according to an embodiment of thepresent disclosure. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Inanother embodiment, as shown in FIG. 4 , the optical signal into thewaveguide 201 received from the input port 200 needs to be split to oneor more branches for many reasons. At least for the case that theoptical signal received from the input port 201 is a polarizationsensitive signal which combines both a partial signal in TE mode and apartial signal in TM mode, a polarization splitter 250 is employed tosplit the partial signal in TE mode in one branch 251 and the partialsignal in TM mode in another branch 252. Optionally, the splitter 250 isa polarization rotation splitter so that the partial signal in TM modein branch 252 is also rotated to a partial signal in TE mode forfacilitating power monitor of original TM mode component by a photodiodesince the photodiode is generally adapted for detecting optical signalin TE mode.

In the embodiment of present invention, a pair of photodiodes withnearly redundant physical property is fabricated in the siliconphotonics substrate 10 for detecting the optical signal received fromthe input port 200 especially for application where the optical power ofthe signal is low. As shown, two tap couplers 301 and 302 arerespectively coupled with the two split branches, a first one 251 and asecond one 252 to tap 5% power of respective partial signals in TE mode(note, the TM mode has been rotated to TE mode). The pair of photodiodesincludes a main PD 461 or PD1 and a dummy PD 462 or PD2. Optionally, thepair of photodiodes PD1 and PD2 are formed in close proximity with adistance less than 500 nm via a same CMOS fabrication process in thesilicon photonics substrate 10 so that PD1 and PD2 have nearly redundantphysical properties such as dark current or resistance across the diodewith similar values or similar variations upon a common cause likeenvironment temperature, bias condition, and operation lifetime.

Unlike conventional approach, the invention provides a novel method forconfiguring the photodiodes for detecting optical signal in low power.In the embodiment as shown in FIG. 4 , the main PD 461 or PD1 isconfigured to optically couple with a first taping output port 255 of afirst tap coupler 301 in the first split branch 251 and at the same timecouple with a second taping output port 257 of a second tap coupler 302in the second split branch 252. While, the dummy PD 462 or PD2 is notoptically coupled to any waveguide branches but is electrically coupledto the main PD 461 in series, anode to cathode, in an electricalsub-circuit for detecting the optical power received from the input port200. An anode of the main PD 461 is connected to a cathode of the dummyPD 362. A current is drawn from the anode 471 of the main PD 461 withthe pair of photodiodes in reversed bias condition. Based on the opticalconfiguration of the embodiment shown in FIG. 4 , one photodiode, themain PD 461 alone, is configured to absorb 5% light from both the firstbranch 251 and the second branch 252 to have nearly doubled valuecomparing to the case of FIG. 2 where light from only one branch isabsorbed by just one photodiode. Additionally, based on the electricalconfiguration of the embodiment, the current I drawn from the anode 471of the main PD 461 will be I_(ph)+ΔI_(dk)=I_(ph)+I_(dk1)−I_(dk2), whereI_(ph) is photocurrent converted from absorption of light tapped fromboth the first branch 251 and the second branch 252, I_(dk1) is a darkcurrent existed in the absorption layer of the main PD under a reversedbias, and I_(dk2) is a dark current existed in the dummy PD under asimilar reversed bias. As it is explained above, I_(dk1) issubstantially equal to I_(dk2) as the pair of photodiodes is made to benearly redundant in structure in a close neighbor distance of d via asame CMOS fabrication process on the silicon photonics substrate 10.Therefore, the effect of dark current variation, due to increasingenvironmental temperature over long-time operation of the siliconphotonics integrated circuit with which the pair of photodiodes isassociated, can be substantially minimized. In fact, the dark currentvariation ΔI_(dk) between two neighboring PDs is very small in an orderof 10⁻² to 10⁻³ μA at a reversed bias of −0.6V over 9000 hours. Thus, amajor noise is substantially reduced in photocurrent measurement whichis directly related to optical power of the optical signal received fromthe input port 200.

In an alternative embodiment, the pair of nearly redundant photodiodescan be configured, as shown in FIG. 5 , to optically couple the main PD(PD1) to one or more tap couplers (referred as Tap1, Tap2, . . . ) toreceive light of the optical signals from one or more branches of aninput waveguide and electrically independently laid in the electricalsub-circuit with the dummy PD (PD2) but under a same reversed biascondition for detecting the optical power. Thus, the PD1 is able toconvert all light power of the optical signal being split into the oneor more branches to a photocurrent I_(ph). When the electricalsub-circuit is operated with both PD1 and PD2 under a same reversed biasvoltage V, a first current I₁=I_(ph)+I_(dk1) is drawn from anode 481 ofPD1 and a second current I₂=I_(dk2) is drawn from anode 491 of PD2,where Idk1 and Idk2 are respective dark currents associated with the PD1and PD2 under the reversed bias voltage V. Both the first current I1 andthe second current I2 can be collected and saved in memory and afirmware can be employed to deduce a differential value I₁−I₂, which isI_(ph)+ΔI_(dk). As the dark current variation ΔI_(dk) is substantiallyminimized by making the PD1 and the PD2 nearly redundant in physicalproperties, e.g, by fabricating the pair of PDs in close neighborhood onthe silicon photonics substrate 10. Again, this method also achievesaccurate detection of a low-power optical signal, especially thepolarization-sensitive signal in coherent photonics system, withsubstantially enhanced sensitivity and reduced noise. This eliminatescostly work on developing highly advanced individual photodiode withhigh sensitivity for detecting low-power optical signal. It onlyrequires the pair of photodiodes to be formed in a close neighboringdistance in a normal CMOS fabrication process.

FIG. 6 shows a flow chart to illustrate a method of forming a pair ofnearly redundant photodiodes for detecting low-power optical signalaccording to an embodiment of the present disclosure. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown, the methodincludes a step of providing an optical input waveguide including one ormore branches in a silicon photonics substrate. The input waveguide isconfigured to receive an input optical signal. Optionally, the inputwaveguide is a silicon-based waveguide. Optionally, the input opticalsignal is polarization sensitive signal including a mixture of a TE-modecomponent and a TM-mode component. An optical splitter is coupled to theinput waveguide to split the input optical signal to respective one ormore branches. Each branch is a silicon waveguide formed in the siliconphotonics waveguide. Optionally, the optical splitter is a silicon-basedwaveguide-like splitter formed in the same substrate. Optionally, theone or more branches includes a first branch and a second branch.Optionally, the first branch receives a TE-mode optical signal withpartial power of the input optical signal that is split by the opticalsplitter and the second branch receives a TM-mode optical signal withrest partial power of the input optical signal. Optionally, the opticalsplitter is provided as a polarization splitter rotator so that theTM-mode optical signal in the second branch has been converted toTE-mode optical signal for facilitating optical detection by photodiodein silicon photonics platform. Optionally, the input optical signal istransmitted with low-power in many high-speed data communicationapplications. Optionally, the input waveguide or at least each splitbranch of the input waveguide includes a tap coupler in siliconphotonics platform formed in the same substrate to tap out a smallpercentage (˜5%) of optical power into a taping output port fordetecting the input optical signal (or the partial signal in each splitbranch).

Referring to FIG. 6 , the method includes a step of forming a pair ofnearly redundant photodiodes in silicon photonics platform in thesilicon photonics substrate. Optionally, the silicon photonics substrateis a silicon-on-insulator (SOI) substrate having a silicon device layeron a buried oxide layer. Optionally, the photodiode in silicon photonicsplatform is a Ge-on-Si photodiode having a strained germanium layerserving an optical absorption layer over the silicon device layer of thesilicon-on-insulator substrate wherein a PIN or PN junction can beformed at least partially in the silicon device layer to take thereversed bias for driving the photo-converted carrier charges to becomea photocurrent to the anode of the photodiode. Optionally, the formationof the Ge-on-Si photodiode is realized via a CMOS fabrication process onthe SOI substrate. Optionally, multiple Ge-on-Si photodiodes of a samedesign, geometry, doping characteristics can be formed in a same CMOSfabrication process. In other words, a pair of nearly redundantphotodiodes can be formed in the CMOS fabrication process. At least, thepair of nearly redundant photodiodes can be a neighboring pair ofphotodiodes formed in the process such that the layer thickness, dopingcharacteristics, and other mechanical or electrical properties aresubstantially the same between them, which leads to nearly the samephysical properties like characteristic dark current under a same biascondition across the junction of the photodiode.

Referring again to FIG. 6 , the method further includes a step ofcoupling a first one of the pair of nearly redundant photodiodesoptically to one or more taping output ports respectively coupled fromthe one or more branches. Optionally, the pair of nearly redundantphotodiodes is formed near the one or more taping output ports.Optionally, the one or more taping output ports includes a first tapingoutput port of a first tap coupler coupled to the first split branch ofwaveguide which carries partial optical signal in TE-mode and a secondtaping output port of a second tap coupler coupled to the second splitbranch of waveguide which carries partial optical signal in TM-mode.Optionally, the splitter is also a polarization rotator which is able toconvert the partial signal in TM-mode in the second split branch to apartial signal in TE-mode. The first one of the pair of nearly redundantphotodiodes is chosen as a main PD to couple each of the first tapingoutput port and the second taping output port to allow the germaniumabsorption layer to absorb light therefrom. This step naturally enlargesthe power level of the absorbed light comparing to using a single PD todetect light branch one individual branch only. As the input opticalsignal may be transmitted in low-power for many applications, this stepallows the main PD be used to detect optical signal at its maximum powerlevel.

Referring to FIG. 6 , the method additionally includes a step ofcoupling a second one of the pair of nearly redundant photodiodeselectrically in series to the first one of the pair of nearly redundantphotodiodes. The second one of the pair of nearly redundant photodiodesis chosen as a dummy PD. Although it is not coupled to any taping outputport to absorb light from any branch of waveguide, the dummy PD isconfigured in an electrical detection sub-circuit associated with thepair of photodiodes by coupling its cathode to the anode of the main PD.Optionally, the dummy PD is formed in a close-proximity neighborhood ofthe main PD in the silicon photonics substrate so that an electricalconductive wiring between the dummy PD and the main PD can be laidconvenient during the CMOS fabrication process for forming theelectrical detection sub-circuit. Optionally, the dummy PD is a nearlyredundant to the main PD in terms of physical properties so that when areversed bias is applied to the pair of photodiodes each of the dummy PDand the main PD shares the bias voltage nearly equally, but only themain PD is used to absorb light signal.

Referring to FIG. 6 again, the method furthermore includes a step ofdrawing a photocurrent from the first one of the pair of nearlyredundant photodiodes with enhanced optical signal detection sensitivityas dark currents of the pair of nearly redundant photodiodes cancel eachother. The step includes applying a reversed bias voltage across thepair of photodiodes. Since the pair of photodiodes is electricalconnected in series and has nearly redundant physical properties likethe PN junction structural geometry dimension and electrical dopingcharacteristics in respective layers of the PN junction, the first one(the main PD) and the second one (the dummy PD) of the pair ofphotodiodes are substantially equally share the bias voltage. For themain PD under the reversed bias condition is used to absorb light fromthe one or more branches of input waveguide, a current drawn from ananode of the main PD shall include a photocurrent converted from all thelight absorbed by the germanium absorption layer of the main PD. At thesame time, due to the electrical connection in series of the pair ofphotodiodes under the reversed bias condition, the current drawn fromthe anode of the main PD (which is also a cathode of the dummy PD) shallincludes contribution of respective dark currents of the two PDs. Infact, one dark current from the main PD is a positive additive to thephotocurrent while another dark current from the dummy PD is a negativeadditive to the photocurrent. As the physical properties of the pair ofphotodiodes are nearly the same and share substantially equally thereversed bias voltage, the dark current of the dummy PD nearly cancelsthe dark current of the main PD in the current drawn from the anode ofthe main PD. This substantially reduces noise in photocurrentmeasurement as the collected current from the anode by the electricaldetection sub-circuit is substantially close to the true value ofphotocurrent converted from the light absorbed by the main PD.

In an alternative embodiment, the dummy PD is configured to receive acommon reversed bias voltage but otherwise not connected to the main PDelectrically. Therefore, the current I_(m) drawn from the anode of themain PD shall include a photocurrent I_(ph) converted from the lightabsorbed and a dark current I_(dkm) under the reversed bias voltage. Atthe same time, another current I_(d) is drawn from anode of the dummy PDunder the same reversed bias voltage includes only a dark currentI_(dkd) thereof since no light is absorbed by the dummy PD. Both thecurrent I_(m) and the current I_(d) can be collected into a memorydevice of a controller associated with the electrical detectionsub-circuit. A firmware pre-loaded in the controller can conduct acalculation to subtract I_(d)=I_(dkd) from I_(m)=I_(ph)+I_(dkm). Underassumption of the dummy PD is made nearly redundant to the main PD, thedark current I_(dkd) of the dummy PD will be substantially equal to thedark current I_(dkm) of the main PD under a common reversed biasvoltage, the subtraction of I_(m)−I_(d) gives a true value of thephotocurrent I_(ph) as the dark current I_(dkd) cancels the dark currentI_(dkm).

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A circuit for detecting an optical data signal,the circuit comprising: a photonics substrate; a splitter configured toreceive the optical data signal from an input port formed in thephotonics substrate and output a first portion of the optical datasignal and a second portion of the optical data signal; a first tapcoupler configured to receive the first portion of the optical datasignal; a second tap coupler configured to receive the second portion ofthe optical data signal; a first photodiode formed in the photonicssubstrate, the first photodiode configured to (i) receive, via the firsttap coupler, the first portion of the optical data signal, (ii) receive,via the second tap coupler, the second portion of the optical datasignal, and (iii) convert light power of the first portion and thesecond portion to generate a first current; and a second photodiodeformed in the photonics substrate without being coupled to any inputport or waveguide, the second photodiode being configured to output asecond current without receiving any portion of the optical data signal,the second current corresponding to a dark current induced in the secondphotodiode, the circuit being configured to subtract the second currentfrom the first current to generate an output signal corresponding to aphotocurrent of the optical data signal without dark current induced inthe first photodiode.
 2. The circuit of claim 1, further comprising afirst waveguide formed in the photonics substrate and coupled betweenthe input port and the first photodiode, the first waveguide comprisingthe splitter and being configured to supply the optical data signal fromthe input port to the first photodiode.
 3. The circuit of claim 2,wherein the first photodiode is coupled to the first waveguide via thefirst tap coupler.
 4. The circuit of claim 3, wherein the firstphotodiode is coupled to first and second branches of the firstwaveguide, the first photodiode being configured to receive the firstportion of the optical data signal via the first branch and receive thesecond portion of the optical data signal via the second branch.
 5. Thecircuit of claim 4, wherein: the first photodiode is coupled to thefirst branch via the first tap coupler, and to the second branch via thesecond tap coupler; and the first tap coupler and the second tap couplerare coupled to the input port via the splitter.
 6. The circuit of claim1, wherein the first photodiode and the second photodiode form aclose-neighbor pair.
 7. The circuit of claim 6, wherein the secondphotodiode is located less than 500 nanometers (nm) from the firstphotodiode in the photonics substrate.
 8. The circuit of claim 6,wherein the first photodiode and the second photodiode are formed in thephotonics substrate under a same complementary metal-oxide semiconductor(CMOS) process.
 9. The circuit of claim 6, wherein the dark currentinduced in the second photodiode corresponds to a dark current inducedin the first photodiode.
 10. The circuit of claim 1, wherein the secondphotodiode is coupled in series with the first photodiode such thatcurrent flowing through the first photodiode includes respective darkcurrents induced in the first photodiode and the second photodiode. 11.The circuit of claim 10, wherein the circuit is configured to generatethe output signal at a node between the first photodiode and the secondphotodiode.
 12. The circuit of claim 11, wherein the circuit isconfigured to generate the output signal based on a photocurrent inducedin the first photodiode.
 13. The circuit of claim 1, wherein the secondphotodiode is not coupled to the first photodiode.
 14. The circuit ofclaim 13, wherein the second photodiode has a same reverse biascondition as the first photodiode.
 15. The circuit of claim 14, whereinthe second photodiode is connected to a same reverse bias voltage as thefirst photodiode.
 16. The circuit of claim 15, wherein the output signalcorresponds to a difference between an output current of the firstphotodiode and an output current of the second photodiode.
 17. Thecircuit of claim 16, wherein: the output current of the first photodiodeis a photocurrent induced in the first photodiode and a dark currentinduced in the first photodiode; and the output current of the secondphotodiode is the dark current induced in the second photodiode.
 18. Thecircuit of claim 1, wherein the first portion of the optical data signalis less than an entirety of the optical data signal supplied to theinput port.
 19. A system comprising: the circuit of claim 1; a memorythat stores a difference between sample currents output respectively bythe first photodiode and the second photodiode when a same reverse biasvoltage is applied to the first photodiode and the second photodiode,the difference indicating a variation in respective dark currentsinduced in the first photodiode and the second photodiode; and acontroller configured to calculate, based on the difference, a portionof the first current corresponding to a photocurrent induced in thefirst photodiode by the optical data signal.